1. Field of the Invention
The present invention relates to a method and apparatus for rapidly and non-destructively measuring properties of a cold rolled thin steel sheet in an off-line system, or a method and apparatus for conducting a non-destructive measurement during execution of the step of passing through a sheet in an on-line system.
2. Description of Related Art
A cold rolled thin steel sheet is required to have a high press-formability because it is used for a body of an automobile or in domestic electrical appliances. The cold rolled thin steel sheet is a polycrystalline material, and the press-formability thereof is determined almost by the so-called "aggregate structure".
The press-formability has hitherto been estimated by an X-ray pole figure method or a Lankford value (an r value). The X-ray pole figure method is time consuming, however, since a test piece should be cut out from a cold rolled thin steel sheet and irradiated with an X-ray for a measurement. Further, in this method, since a test piece is cut out purposely, 30 the test method is destructive, and therefore is unsuitable, particularly for an on-line measurement. On the other hand, the method wherein a Lankford value is measured is time consuming, since an accurate measurement on a change in the size of a tensile test piece is necessary. Further, in this method, since a test piece is cut out from the cold rolled thin steel sheet, the measuring method is destructive. In another method, a simple method is proposed (C. A. Stickers and Mould, "The use of Young's modulus for predicting the plastic strain ratio of low carbon steel sheets", Metallurgical Transaction, Vol. 1, pp. 1303-1312 (1970)). Utilizing an empirical correlation between the Young's value measured by a natural polarization method and the Lankford value, but since a test piece is cut out from a cold rolled thin steel sheet, this measuring method is also destructive.
For this reason, a method wherein the velocity of an ultrasonic wave is measured has been proposed wherein a non-destructive measurement is conducted through the use of a cold rolled thin steel sheet per se without cutting out a test piece as in the above-described measuring method. In a method wherein the Young's modulus or Lankford value of a cold rolled thin steel sheet is obtained by the above-described non-destructive measuring method (for example, see Japanese Unexamined Patent Publication (Kokai) No. 2-210258), the above-described K.sub.1 and K.sub.2 values are measured by means of a resonant electromagnetic ultrasonic wave sensor and, at the same time, the ratio, K.sub.3, of the speed of SH0 sheet wave ultrasonic wave, V.sub.SH0 (45.degree.), propagating within the cold rolled thin steel sheet in a direction at 45.degree. to the direction of rolling to the speed of SH0 plate wave ultrasonic wave, V.sub.SH0 (0.degree.) or V.sub.SH0 (90.degree.), propagating in a direction parallel to or normal to the direction of rolling should be measured by means of a SH0 plate wave ultrasonic wave sensor of an electromagnetic ultrasonic wave sensor system. That is, it is necessary to use an resonant electromagnetic ultrasonic wave sensor and a SH0 sheet wave ultrasonic wave sensor. In particular, two pairs of the above-described resonant electromagnetic ultrasonic wave sensors and SH0 plate wave ultrasonic wave sensors should be used as an ultrasonic wave sensor for measuring the K.sub.3 value. Therefore, in this case, there occur problems in that it is difficult to reduce the size of the measuring apparatus and it is difficult to stably generate and detect a plate wave ultrasonic wave when a non-contact measurement is conducted in an on-line system. The term "SH (shear horizontal) plate wave" used herein is intended to mean a transverse wave that is parallel (horizontal) to the boundary surface of a thin steel sheet or the like and causes a particle to be polarized in a direction orthogonal to the direction of advance. The above-described SH0 wave is one of the SH plate waves and is intended to mean a 0 (zero) mode wave.
In a patent directed to an apparatus for evaluating a drawability of a metallic thin sheet (for example, Japanese Unexamined Patent Publication (Kokai) No. 1-214757), a S.sub.0 mode ultrasonic wave is propagated in directions of 0.degree., 450.degree. and 90.degree. in the direction of rolling, and the Lankford value is obtained by measuring the time taken for the ultrasonic wave to propagate a given distance. The drawability is also referred to as a press-formability, which is an important factor in a metallic thin sheet, for example, a body of an automobile. Usually, the drawability is estimated on the basis of Lankford values. In this method, however, three pairs of electromagnetic ultrasonic wave sensors for transmission and an electromagnetic ultrasonic wave sensor for receiving are necessary, so that the size of the whole measuring apparatus is large, that is, 30 cm.times.30 cm or more, and therefore is not very convenient.
In other patents (for example, Japanese Unexamined Patent Publication (Kokai) No. 64-83322), the velocities of three types of ultrasonic waves are measured, and the ratio, K, of the average velocity of two transverse waves to the velocity of a longitudinal wave is calculated. This patent demonstrates that the K value is related to the coefficient of crystal orientation, W.sub.400, and the in-plane average value of Lankford values. In effect, after a secondary regression formula of the K value and the in-plane average value of the Lankford value is determined from an empirical formula, the in-plane average value of the Lankford value is estimated from the above-described K value. In this method, however, since use is made of the ratio of the velocity of a longitudinal wave to the average velocity of sound of two transverse waves, the number of data is only one. Further, since the coefficients of crystal orientation, that is, W.sub.420 and W.sub.440, are not taken into consideration, it is believed that the precision of the calculation is lowered. Further, according to working examples of the above-described patent, a metallic thin sheet is brought into contact with an ultrasonic wave probe to conduct the measurement, which renders the above-described method unsuitable for a measurement in an on-line system.